I'm happy to engage in discussion and am not offended by blunt questions -- I was in the regular army infantry for eleven years of active duty (where concern over giving offense during verbal exchanges and "conversations" was not common) and subjected to the system at West Point for four years before that (where concern over giving offense during verbal exchanges and "conversations" was also not common).
I would say that phrases such as "give you the benefit of the doubt" could be interpreted as something of a threat, as is the phrase in the above response in which you say "as long as you don't" -- these types of phrases imply the veiled (or thinly veiled) threat of consequences to those you are addressing -- so forgive me if I take umbrage at phrases that are commonly used by those communicating a threat. Perhaps you did not intend your expressions to convey that sense.
I believe that the myths you are calling "agrarian and creation myths" from around the world are actually profound spiritual metaphors of the same exalted character and origin as the "agrarian and creation myths" collected into what we today call the Bible. Myths such as the Osiris myth or the Persephone myth, often interpreted as being primarily about the annual cycle of the seasons and the death and return of the grain and the other growing things, are not primarily about that at all, but rather about our condition as incarnate souls in a world which is apparently material but which is completely at every point interpenetrated by a spiritual realm (which is indeed referred to as the "seed realm" in some of the ancient metaphors) and which (like the sleeping seed beneath the soil) is not seen but full of potential (or "potentiality").
So to answer your questions most directly, I would say that I believe that whoever gave humanity these myths already had a very clear idea of the spiritual truths they wanted to convey when he/she/or they decided to use the "canvas of the stars" to help us to understand them.
Who gave them to humanity I believe is an open question and I do not have the answer -- but I think that the possibilities include a) humans with exalted spiritual understanding, b) supernatural divinity or divinities, or c) other beings (take your pick of where these may have come from -- other dimensions, other planets, this planet, etc). And that's not an exhaustive list, of course.
I do NOT believe that the possible answer includes: a) primitive humans trying to explain the crop cycles b) primitive humans trying to explain the source of thunder or the literal origin of fire, etc. The reason I don't believe those possibilities is that the depth of these metaphors and the exalted spiritual message that is contained in them is unlikely to have been a later addition to "primitive explanations of the natural world" -- it is very unlikely that such "primitive explanations" just happened to, by a happy accident repeated all around the world by the authors of other "primitive explanations," work out nicely to convey exalted spiritual truths when "later less primitive" people came along and decided to import spiritual truths on top of old "primitive natural-world explanations."
I would liken the use of the incredible metaphors of the stars to the use of the vast ocean and the great whales by Herman Melville in Moby Dick.
He was searching for a "canvas" that would be "big enough" to enable him to portray the vast and profound subjects which he wanted to wrestle with in that novel. In the vast depths and the brutal dangers and difficulties of life on a whaling ship and the awesome majesty of the great cetaceans, he found a canvas that was big enough to "cloak in flesh and matter" questions about fate, destiny, providence, mercy, grace, and the mystery of whether we are here for any reason at all or no reason.
I believe that whoever gave humanity the treasure of the ancient myths uses the celestial "ocean" in very much the same way.
I write about that a little in a blog post here (published on a past anniversary of the publication of Moby Dick): http://mathisencorollary.blogspot.com/2014/11/happy-anniversary-to-moby-dick.html
I hope that addresses the questions you've asked, which I believe are both valid and insightful questions.
If not, ask them again! I'll address them eventually if I am able to do so!